Waterjet cutting apparatus has been used for some years to cut a variety of materials such as steel, aluminium, glass, marble, plastics, rubber, cork and wood. The work piece is placed over a shallow tank of water and a cutting head expelling a cutting jet is accurately displaced across the work piece to complete the desired cut. The cutting action is carried out by the combination of a very high pressure jet (up to 90,000 psi) of water entrained with fine particles of abrasive material, usually sand, that causes the cutting action. The water and sand that exit the cutting head are collected beneath the work piece in the tank.
It is in the industry associated with waterjet cutting that the expression “ultra high pressure” (UHP) waterjets are used to define a process where water is pressurised above 50,000 psi and then used as a cutting tool. The high pressure water is forced through a very small hole which is typically between 0.1 mm and 0.5 mm in diameter in a jewel which is often ruby, sapphire or diamond.
Although pressures greater than 50,000 psi are defined as ultra high pressure it is envisaged that these pressures could be as great as 100,000 psi.
In our co-pending patent application WO 2009/117765 we disclose an ultra high pressure pump that has been specifically designed for use with a particular type of waterjet cutting apparatus. The issues of compactness and efficiency are critical to pumps of this nature and there is a need for pump to operate reliably at ultra high pressures. There is also a need for the pumps to be designed in a manner that they can be readily fitted to many types of existing waterjet cutting machines. There is also a need for the pumps to regulate the pressure accurately with minimal pressure variation.
It is these issues that have brought about the present invention.